1. Field of the Invention
This invention relates to an optical wavelength converting apparatus for converting a laser beam, which serves as a fundamental wave, into its second harmonic, or the like. This invention particularly relates to an optical wavelength converting apparatus, wherein a semiconductor laser is utilized as a light source for producing a fundamental wave.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the fundamental wave of a laser beam into its second harmonic, or the like, e.g. to shorten the wavelength of a laser beam, by using a nonlinear optical material. In such techniques, semiconductor lasers are often utilized as light sources for producing fundamental waves, which are to be converted into their second harmonics, or the like.
The laser beam, the wavelength of which has been shortened, is often utilized in a color laser printer, an optical pickup device, or the like. In many cases, it is desired that the laser beam be generated in a single longitudinal mode. Some of semiconductor lasers, which are utilized as light sources for producing fundamental waves, generate laser beams in the single longitudinal mode. The requirement described above can be satisfied by using such semiconductor lasers.
However, in general, semiconductor lasers, which generate laser beams in the single longitudinal mode, exhibit only a low level of output power. Therefore, in cases where such semiconductor lasers are used, a wavelength-converted wave having a high output power cannot be obtained.
On the other hand, various semiconductor lasers, such as broad area lasers and phased array lasers, have heretofore been proposed which generate laser beams in a multiple longitudinal mode and which can exhibit a high output power. Also, as described in, for example, Appl. Phys. Lett., Vol. 50, No. 24,15, June 1987, p. 1713, a technique has been proposed wherein a laser beam produced in a single longitudinal mode is injected into a semiconductor laser capable of oscillating in a multiple longitudinal mode, and a laser beam having the same wavelength as the injected laser beam is thereby obtained from the semiconductor laser capable of oscillating in the multiple longitudinal mode. Such a technique is referred to as injection locking. It is considered to apply such a technique to multiple-longitudinal-mode semiconductor lasers, which are used as light sources for producing fundamental waves.
However, in cases where the aforesaid injection locking technique is applied to multiple-longitudinal-mode semiconductor lasers, which are used as light sources for producing fundamental waves, the problems described below often occur. Specifically, the wavelength of the laser beam, which is generated by the semiconductor laser for injection locking, fluctuates. Therefore, the wavelength of the laser beam, which is generated by the multiple-longitudinal-mode semiconductor laser, also fluctuates. As a result, the intensity of the wavelength-converted wave cannot be kept stable.